The Bishop of Durham has been unveiled as the new Archbishop of Canterbury and said being nominated for the post was "astonishing and exciting".

The Rt Rev Justin Welby, an Eton-educated former oil industry executive, will become the 105th Archbishop and spiritual leader of the 77 million-strong Anglican Communion.

Prime Minister David Cameron welcomed the appointment, saying he "wished him success in his new role".

Bishop Welby told a news conference at Lambeth Palace: "This is the best-kept secret since the last Cabinet reshuffle. To be nominated to this post is both astonishing and exciting."

Bishop Welby was the fourth most senior cleric in the Church of England before the announcement that he would succeed Rowan Williams.

No 10 made the unusual move of announcing the appointment on social networking site Twitter, stating: "Downing Street is pleased to announce the appointment of Justin Welby as the 105th Archbishop of Canterbury."

The bishop began his press conference with a prayer and said: "It's something I never expected, and the last few weeks have been a rather strange experience, to put it mildly."

He added: "One of the biggest challenges is to follow a man who I believe will be recognised as one of the greatest Archbishops of Canterbury, Rowan Williams."

And he joked: "On the basis that you should only follow failures, this is a great mistake."

The appointment marks a meteoric career rise for the clergyman who was enthroned as Bishop of Durham only a year ago. He worked in the oil industry for 11 years before leaving to train for the Anglican priesthood, and was first ordained as a deacon in 1992.